1. Field of the Invention
The present invention relates to a vacuum pump, and more particularly to a vacuum pump which is preferably used for evacuating a gas from a vacuum chamber used in a semiconductor fabrication process.
2. Description of the Related Art
There has heretofore been known a vacuum pump called a positive-displacement pump having a pair of pump rotors which rotate synchronously with each other for drawing in and discharging a gas. The positive-displacement vacuum pumps include Roots vacuum pumps, screw vacuum pumps, and the like, and are widely used for evacuating a process gas from a vacuum chamber used in a semiconductor fabrication process.
FIG. 1 of the accompanying drawings is a cross-sectional view showing a conventional Roots-type positive-displacement vacuum pump which comprises a multistage Roots vacuum pump having a pair of pump rotors housed in a casing. Each of the pump rotors has three-stage Roots rotors. As shown in FIG. 1, a pump rotor 51 comprises rotors 51a, 51b and 51c disposed in a casing 52, and a main shaft 51d rotatably supported by bearings 53. The vacuum pump has an inlet port 54 formed in the casing 52 and disposed above the rotor 51a which has the largest axial width, and has an outlet port 55 formed in the casing 52 and disposed below the rotor 51c which has the smallest axial width. The pump rotor 51 is coupled to a motor M at the end of a main shaft 51d located at the outlet port side. A timing gear 56 is fixed to the pump rotor 51 at the other end of the main shafts 51d located at the inlet port side. The timing gears 56 serves to link a pair of confronting pump rotors 51, one of which is shown in FIG. 1.
When the motor M is energized, any pair of pump rotors 51 are synchronously rotated in opposite directions by the timing gears 56, and a gas is delivered and evacuated by the multistage rotors 51a, 51b and 51c. When the multistage vacuum pump is in operation, the rotor 51a positioned near the inlet port 54 is not heated to a high temperature, but the rotor 51c positioned near the outlet port 55 is heated to a high temperature by the gas which has been compressed by rotors 51a, 51b and 51c in a multistage manner.
A motor used for driving a vacuum pump generally comprises an induction motor. The motor M comprises a motor rotor 57 having an iron core 58 which consists of a laminated assembly of electromagnetic steel sheets. The iron core 12 has slots accommodating rotor bars (not shown) as secondary conductors. When the motor M is in operation, induced current flows through the secondary conductors due to a rotating field produced by a stator 60 of the motor M. Thus, the rotating field and the induced current generate a torque for rotating the motor rotor 57. However, the induced current flowing through the secondary conductors heats the motor rotor 57, thereby making the motor M itself high in temperature.
In the case where the motor is disposed at the outlet port side heated by the compressed gas and with the motor M itself generating heat, the motor M tends to be excessively heated to a high temperature and hence greatly reduced in efficiency. Therefore, in the case where the motor is disposed at the outlet port side, the motor M needs to be connected to the main shaft 51d through a coupling or the like so that the motor M is spaced by a certain distance from the outlet port 55. The motor M having such a structure causes the vacuum pump to be more susceptible to a mechanical loss, and also fails to make the vacuum pump compact.
Another conventional vacuum pump has a motor which is disposed at an inlet port side having a relatively low temperature and has a motor rotor which comprises permanent magnets directly fixed to a pump rotor and generates no induced current. However, if the vacuum pump draws in a corrosive gas used in the semiconductor fabrication process, the permanent magnets of the motor are corroded by the gas. Consequently, the permanent magnets reduce their magnetic forces and are damaged, possibly resulting in a motor failure. If some of permanent magnets are damaged while the motor is being assembled, then the permanent magnets tend break and scatter while the motor is in rotation, thus causing the vacuum pump to stop. Furthermore, when the motor rotor is suddenly heated to a high temperature, the mechanical strength of the permanent magnets is lowered and/or the bonding strength of an adhesive by which the permanent magnets are held in place is reduced. Consequently, the permanent magnets detach from the motor rotor during rotation, resulting in a breakdown of the motor.
In the vacuum pump where the motor is located at the inlet port side, it is necessary to dispose a bearing for supporting the motor at the inlet port side. In the case where the bearing is disposed at the inlet port side, oil molecules from grease used to lubricate the bearing are likely to diffuse into a region located upstream of the inlet port where a vacuum is developed. Therefore, a shaft seal mechanism is required to be installed to prevent the oil molecules from diffusing into the vacuum chamber. The shaft seal mechanism makes it difficult to simplify the structure of the vacuum pump, and also makes it difficult to make the vacuum pump compact. These problems occur also in other positive-displacement vacuum pumps such as a screw vacuum pump.